Optical element and system using the same

ABSTRACT

A method for transmitting a signal in an optical system includes generating an optical signal along an optical axis for transmission through an optical element, positioning the optical element so that a surface discontinuity is positioned along the optical axis such that the optical signal defines a substantially radially symmetric intensity profile, and launching the optical signal into an input face of an optical fiber such that the intensity profile is substantially null proximate an optical axis associated with the optical fiber.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a division of pending application Ser. No.12/805,478, filed Aug. 2, 2010, which in turn is a continuation-in-partof application Ser. No. 12/453,229, filed May 4, 2009, now U.S. Pat. No.7,769,258 on Aug. 3, 2010, which is a continuation of application Ser.No. 12/073,748, filed Mar. 10, 2008, now U.S. Pat. No. 7,529,446, whichin turn is a continuation of Ser. No. 11/802,044, filed May 18, 2007,now U.S. Pat. No. 7,343,069 B2, which in turn is a continuation of Ser.No. 10/320,525, filed Dec. 17, 2002, now U.S. Pat. No. 7,221,823 B2,which is a continuation of Ser. No. 09/614,184, filed Jul. 11, 2000, nowU.S. Pat. No. 6,496,621, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/329,996, filed Jun. 11, 1999, now U.S. Pat. No.6,530,697, which claims priority under 35 U.S.C. §119 to ProvisionalApplication No. 60/101,367 filed on Sep. 22, 1998, the entire contentsof all of which are hereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention are directed to an optical elementhaving both an amplitude diffractive structure and a phase diffractivestructure and/or a vortex lens.

2. Description of Related Art

It is very difficult to manufacture a multi-mode fiber with good controlover the index of refraction in the center of the fiber. If the lightcoupled to the fiber excites some modes that propagate mostly in thecenter of the fiber and other modes which do not propagate mostly in thecenter of the fiber, very different propagation times for these modesmay result. This is referred to as differential mode delay. Differentialmode delay tends to spread out the pulse length of signals and reducethe effective bandwidth of the fiber.

Modes which propagate mostly in the center of the fiber are the lowerorder fiber modes, i.e., modes having small propagation angles thatstrike at or near the center of the fiber. These lower order modes spendmost of the time in the center of the fiber, tend to travel straightdown the fiber and the shape of these modes does not change much as theypropagate. Therefore, in order to reduce differential mode delay, anylight which enters near the center of the fiber needs to be incident atan angle which is large enough not to excite lower order modes, but notso large that the critical angle is exceeded and the light fails to becoupled or no light should be input to the center of the fiber.

One current solution involves coupling light into single mode fiberswhich are then positioned off-axis relative to the multi-mode fiber.Single mode fibers have a much smaller core than multi-mode fibers, socan be used to provide light at specific positions on the endface of themulti-mode fiber. However, single mode coupling is more expensive thanmulti-mode coupling and the additional coupling step leads to anincrease loss in light. Further, while no light enters the fiber of thecenter for this configuration, the light will still cross the fiber axisas it propagates, thus increasing the differential mode delay.Additionally, ferrules or other structures housing the multi-mode fiberto a single mode fiber junction are not readily available and must bedeveloped specifically for that purpose.

Another solution is to use a vertical cavity surface emitting laser(VCSEL) excited to radiate in a ring mode. The operation of the VCSEL inradiation modes other than the lowest order have less predictable fluxdistributions than in the lowest order mode, in which the distributionmore closely approximates a Gaussian profile. Further, there will stillbe some power in the lower order modes of the VCSEL. Additionally, suchoperation of the VCSEL often requires a higher current to drive thesource into the higher radiation modes.

As the use of non-physical contact connections between light sources andfibers increases, the need for effective isolation to prevent lightreflected at the fiber interface from being returned to the light sourceincreases. Feedback to the light source may result in spectralbroadening, light source instability, and relative intensity noise,which affect the monochromaticity of the light source. As data rates goup, the systems become more sensitive to relative intensity noise andrequire low bit error rates. Conventional optical isolators usingpolarization effects to attenuate reflection are very expensive, makingthe non-physical contact impractical. The importance of avoidingfeedback is further increased when trying to use cheaper light sources,such as vertical cavity surfaces emitting laser diodes and lightemitting diodes.

One solution that avoids the use of an optical isolator is a modescrambler that divides power from the light source into many modes. Aconfiguration employing a mode scrambler includes a single mode pigtailthat provides light from the light source to the mode scrambler thatthen delivers the light to a transmission cable via an air-gapconnector. Since any reflected power will still be divided across themany modes, any reflected power in the mode that can efficiently becoupled into the pigtail is only a small fraction of the total reflectedpower, thereby reducing return losses. However, this solution involvesaligning another fiber, physically contacting the fiber with the modescrambler, and placing the light source against the fiber. Thispigtailing is expensive. Thus, there still exists a need for truenon-physical contact connection between a light source and atransmission system that does not require an isolator.

SUMMARY

Embodiments are therefore directed to an optical element thatsubstantially overcomes one or more of the problems due to thelimitations and disadvantages of the related art.

Embodiments may be directed to providing a method for transmitting asignal in an optical system, the method including generating an opticalsignal along an optical axis for transmission through an opticalelement, positioning the optical element so that a surface discontinuityis positioned along the optical axis such that the optical signaldefines a substantially radially symmetric intensity profile, andlaunching the optical signal into an input face of an optical fiber suchthat the intensity profile is substantially null proximate an opticalaxis associated with the optical fiber.

An axicon function may be provided with respect to the optical signalvia the surface discontinuity.

The relative minimum of the incident field amplitude of the intensityprofile may be located proximate a center of the intensity profile.

The intensity profile may be substantially symmetric about the relativeminimum of the incident field amplitude.

The intensity profile may be substantially annular.

Embodiments may be directed to providing an optical system including anoptical element having an input surface, an output surface, and anoptical axis, and an optical medium having an end proximate to theoutput surface of the optical element and an end distal to the outputsurface of the optical element, the optical medium configured to receiveemitted light having a received annular intensity profile and transferthe light to the distal end such that the transferred lightsubstantially retains the received annular intensity profile at thedistal end. The output surface may have a continuous slope/curvature inan area approximately between a periphery of the output surface and afirst distance from the optical axis. The output surface may have adiscontinuity in an area within a perimeter of the first distance foremitting light having an output annular intensity profile receivable atthe proximate end of the optical medium.

The optical element may be a mode matching element, wherein light outputfrom the optical element is distributed in a desired angulardistribution which is substantially maintained along the fiber for morethan a depth of focus of the optical element.

The mode matching element may be a diffractive element or a refractiveelement.

The optical element may include first and second surfaces, the modematching element being provided on said second surface, further from thelight source. An angular distribution altering element may be on thefirst surface. The angular distribution altering element may provide aring pattern on said second surface. The angular distribution alteringelement may be a diffractive surface having a radially symmetric lensfunction and a negative axicon function.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages will become more apparent tothose of ordinary skill in the art by describing in detail exemplaryembodiments with reference to the attached drawings, in which:

FIG. 1 illustrates the integration of the coupler of an embodiment witha light source, a fiber and a light source power monitor;

FIGS. 2A-2C illustrate a diffractive element and associatedcharacteristics of a spiral generator for use as the coupler inaccordance with an embodiment;

FIG. 3 is a schematic illustration of another embodiment of the coupler;

FIG. 4 is a schematic illustration of another embodiment of the coupler;

FIG. 5A illustrates a schematic view according to an embodiment;

FIG. 5B illustrates a schematic view according to an embodiment;

FIG. 6 illustrates a radiation profile of light traversing an embodimentof FIG. 5A or 5B;

FIG. 7 illustrates another embodiment of the optical element; and

FIGS. 8A, 8B and 9 illustrate other embodiments of the optical elementand/or beam shaper.

DETAILED DESCRIPTION

FIG. 1 illustrates a light source 10, here a VCSEL, a coupler 12 and amulti-mode fiber 14 integrated with a power monitor 16 and a reflectivesurface 18 for directing the light into the fiber 14. In particular, thelight source 10 and the power monitor 16 are provided on a substrate 20.Another substrate 22 has the coupler 12 thereon, preferably on the facefurthest from the light source to allow the beam to expand, and asplitting diffractive element 24 which splits off a portion of the lightfrom the light source 10 to be monitored. The substrates 20, 22 arepreferably mounted with spacer blocks 26, which provide the desiredseparation between the substrates 20, 22. The coupler 12 may also beprovided in a common housing with the fiber 14.

The light split off by the diffractive element 24 is directed to thepower monitor 16 to monitor the operation of the light source 10. Thedirected of the light to the power monitor 16 may be achieved byproviding appropriately positioned reflective portions 28. The number oftimes the light to be monitored traverses the substrate 22 is a designchoice, depending on the initial angle of diffraction and the desiredpositioning of the power monitor 16. This monitoring is set forth incommonly assigned U.S. application Ser. No. 09/386,280, entitled “ADiffractive Vertical Cavity Surface Emitting Laser Power Monitor andSystem” filed Aug. 31, 1999, which is hereby incorporated by referencein its entirety for all purposes. Alternatively, the power monitoringmay be realized using an integrated detector, without the need for thedeflecting element, as set forth in commonly assigned U.S. applicationSer. No. 09/548,018, entitled “Transmission Detection for VerticalCavity Surface Emitting Laser Power Monitor and System” filed Apr. 12,2000, which is hereby incorporated by reference in its entirety for allpurposes

The light that is not split off by the diffractive element 24 proceedsto the coupler 12. A reflective surface 18, such as a polished angularface of another substrate, is provided to direct the light from thecoupler 12 into the multi-mode fiber 14. Preferably all the opticalelements are formed lithographically and all the elements are integratedon a wafer level.

In accordance with the present invention, the coupler 12 is adiffractive element that matches the phase as well as the intensitydistribution of the beam. The matching of the phases generates spiralpropagation of the beam through the fiber. This spiral or vortexpropagation maintains the intensity profile input to the fiber along thefiber. Since the beam travels in a corkscrew, the amount of lightcrossing the center of the fiber is significantly reduced. Ideally, theamount of light in the center will be zero, but in practice, the amountof light is on the order of 10% or less. In contrast, when only theintensity distribution is controlled, as in the first two designs of theparent application, the input intensity profile may be the desiredprofile, but will quickly degrade as the light traverses the fiber, Inother words, while the other designs may provide an input profile thatis substantially null on axis, this profile is only maintained for thedepth of focus of the coupler. When also matching the phase, thisprofile is maintained substantially beyond the depth of focus of a lenshaving the same numerical aperture as the beam to be input to the fiber,e.g., at least an order of magnitude longer. Absent the fiber, the nullspace of the beam profile is maintained through free space, whichsignificantly reduces the alignment requirement. Further, by matchingthe phase and amplitude of the beam to a certain mode of the fiber,theoretically the beam profile could be maintained over an infinitelength of fiber. However, imperfections in the real world, e.g., in thefiber, in the beam, in the matching, degrade from this theoreticalscenario.

Thus, in order to avoid low order modes in a GRIN fiber launch, theamplitude and phase of the higher order modes need to be matched. Thefollowing equations are set forth in Fields and Waves in CommunicationElectronics, Simon Ram et al. 1984, particularly pp. 765-768, which ishereby incorporated by reference in its entirety. For a GRIN fiber,these eigenmodes all have the form set forth in Equation (1):

E(r,θ,z)∝ƒ_(mp)(r)e^(±jmθ)e^(±jβ) ^(mp) ^(z)   (1)

where ƒ(r) is a function that depends only on r for given modes within aspecific fiber, r is the radius from the axis, θ is the angle from theaxis, z is the distance along the axis, m is the azimuthal mode number,β is a propagation constant, p is the radial mode number. When m, p=0,the beam has a Gaussian profile.

While Equation (1) could be used to match a particular mode of the fiberby creating an input light beam having an amplitude and phase functionwhich exactly correspond to the particular mode, such matching is notrequired and may not even be desirable, as matching the amplitude aswell as the phase increases the requirements on the optics. As long asm>0, the azimuthal mode m will have a phase function that is a spiralring, whether the light is traveling in free space or in a fiber. Oncethe phase function for at least one higher order mode, i.e., m>0, hasbeen matched, a null at the center of the beam is created after the beamhaving been phase matched propagates over a short distance, e.g., a fewwavelengths. Unlike other types of matching, this null is maintained inthe center in both free space and the fiber, so such an optical elementproviding such matching does not have to be immediately adjacent to thefiber. As evident from Equation (1), when matching the phase, the valueof p doesn't matter.

In order to suppress the lowest order mode, i.e., m=0, a phase termneeds to be added to the wavefront. This is accomplished through the useof the following diffractive phase function encoded onto the wavefrontset forth in Equation (2):

$\begin{matrix}{{\varphi \left( {x,y} \right)} = {m\; {\arctan \left( \frac{y}{x} \right)}}} & (2)\end{matrix}$

where φ is the phase function, x and y are the coordinates in the plane.In general, there will be several modes propagating, e.g., m=1-5. Thespiral mode may be realized by matching the phase function for m=3.

This phase function can be added to a lens function and encoded as amod(2π) diffractive element as set forth in Equation (3):

$\begin{matrix}{{\varphi \left( {x,y} \right)} = {\frac{\pi \left( {x^{2} + y^{2}} \right)}{\lambda \; f} + {m\; {\arctan \left( \frac{y}{x} \right)}}}} & (3)\end{matrix}$

FIG. 2A illustrates the mod(2π) diffractive element and thecorresponding intensity to in the focal plane of the lens function. FIG.2B illustrates an actual example of a diffractive optical element 12created in accordance with Equation (3). FIG. 2C illustrates thesimulated ring intensity 25 and the measured intensity pattern 29 of theelement 12 in FIG. 2B. A refractive equivalent in accordance withEquation (3) of the phase matching diffractive 12 may be alternatelyemployed.

This phase matching coupler 12 is not a true beam shaper, since eachpoint in the input plane is mapped into more than one point in theoutput plane because of the axial singularity. Unlike a diffuser, eachpoint in the input plane is not mapped to every point in the outputplane.

The phase matching coupler 12 allows the desired angular distribution tobe substantially maintained along a portion of the fiber. This may bequantified by measuring the amount of power within a certain radius ofthe fiber at a certain distance along the fiber. The phase matching ofthe present invention allows more power to be contained within thedesired radii for a longer distance than methods not employing phasematching. For example, by aligning the coupler and a GRIN fiber alongthe same axis, using a 850 nm source, and matching both the phase andthe amplitude, the encircled energy can be maintained to less than 12.5%is a radius of less than 4.5 microns and 75% for a radius less than 15microns, with no power in the fiber center, for over 6 m.

By matching the phases, the light from the coupler is input to the fibertraveling in a circular direction, i.e., the path of the light down thefiber forms a corkscrew. Such traversal is opposed to the linear travelnormally occurring down the fiber. By traveling in a corkscrew or spiralmode, the input distribution, typically annular, of the input light ismaintained along the fiber. Without the phase matching, while theinitial input light has the desired shape, this shape is not retainedthroughout the traversal of the fiber. Therefore, more modal dispersionwill be present, with more light in the center of the fiber, if phasematching is not used.

In addition to efficiently coupling the light into the fiber, the phasematching coupler 12 also reduces the power being fedback into the lightsource 10. Since the phases are matched, and the reflected light willnot have the same phase as it did when originally incident on the phasematching coupler 12, the phase matching coupler 12 will not return thelight back to the light source as it came. In other words, when thereflected light traverses the system, it will be further deflected bythe phase matching coupler 12, thereby reducing the power fedback intothe light source 10.

The back reflection reduction of the phase matching coupler onlyoperates sufficiently when the phase matching coupler 12 is far enoughaway from the fiber so that the phase is sufficiently changed to preventbeing redirected in the same manner. In other words, if the phasematching coupler 12 is placed in contact with the end of the fiber,while the coupler will still serve to maintain the input distribution,since the reflected light will have essentially the same phase as theinput light, the light will be returned substantially back to the lightsource as it came. However, if the phase matching coupler 12 is placedat least roughly three times the diameter of the beam incident on thefiber, there is sufficient alteration of the phase due to traversal thatthe reflect light incident on the phase matching coupler 12 will befurther deflected.

Further reductions to the amount of light being fedback to the lightsource 10 may be realized by using a lens 30 in addition to the phasematching coupler 12 as shown in FIG. 3. This lens 30 is used to shapethe light to provide additional reduction in the power fedback to thelight source. The lens 30 is preferably a diffractive surface that is acombination of a lens function having radially symmetric terms with anegative axicon function. When the phase matching coupler 12 is spacedaway from the fiber, the lens 30 may simply form a ring, since the phasematching coupler will prevent the light from being retraced. As shown inFIG. 3, the lens 30 is on a first surface 34 of a wafer 32. The phasematching coupler 12 is provided on a second surface 36 of the wafer 32,opposite the first surface. The thickness of the wafer 32 controls thenumerical aperture of the image. Alternatively, the phase matchingcoupler 12 may be formed on the same surface as the lens 30.

The lens 30 allows an annular intensity ring to be optimized for theparticular fiber 14. Also, by forming this ring prior to the phasematching coupler 12, a smaller radial segment of the phase matchingcoupler is used. As can be seen from equation (2), as m increases, theamount of phase twist increases. Thus, rays at the center of the phasematching coupler 12 receive a larger skew angle that rays at the edge ofthe phase matching coupler. By shaping the light into an annulus, thiscentral portion is avoided, reducing the aberrations introduced by thephase matching coupler 12. Again, the light reflected back from thefiber 14 will not have the same phase as the light incident on the phasematching coupler 12, so the light will be further deflected by the phasematching coupler 12. Since the deflection angles are now altered fromthat of the light source, the lens 30 will not focus the light back ontothe light source, but will further deflect the light away from the lightsource.

Another embodiment is shown in FIG. 4. Here, the phase matching coupler12 is not used, only a reciprocal, phase sensitive system 40. An opticalelement will map an optical distribution, i.e., amplitude and phasedistribution in an input plane to an output plane. If an optical elementis a reciprocal optical, it will map the same optical distribution in anoutput plane back to the original optical distribution in the inputplane, as long as the light has the same phase and intensity profile.Optical systems that perform one-to-one mapping, such as an imaginglens, are reciprocal, but are also phase insensitive when performing amapping between an object plane and an image plane, i.e., a change inphase will not affect the mapping between the image and object planes.However, other optical systems, such as those that perform a one to manymapping, i.e., in which one point in the input plane is mapped to morethan one point in the output plane, while reciprocal, are typicallyphase sensitive. In other words, a phase change will alter how the lightin the output plane is returned to the input plane. An example of such asystem is a negative axicon.

In the preferred embodiment, this system 40 also creates an intensityring on the plane at which the fiber 14 is located. The reflection fromthe fiber creates a ring back onto the system 40, but the phase of thelight has been altered due to the reflection. This change in phaseresults in the light traversing the system 40 having an increaseddiameter of the ring in the object plane, rather than returning the ringto the point source of the light source. This increased diameter resultsin most of the light missing the input of the light source,significantly reducing feedback. Any other reciprocal, phase sensitivesystem that results in most of the light avoiding the light source maybe used. The phase matching coupler 12 may still be employed in anyposition to increase coupling bandwidth and/or enhance the feedbacksuppression.

FIGS. 5A and 5B schematically illustrate different configurations of themulti-mode coupler of the present invention. A light source 110, such asa VCSEL, an edge emitting laser or a single mode fiber, outputs lightwhich is incident on a beam shaper 113 which shapes the beam and anoptical element 112 together forming the coupler. The light is thensupplied to a core 116 of a multi-mode fiber 114.

The end face of the fiber is typically located near the image plane ofthe optical system as determined by the focal length of the beam shaper113 and the object distance, i.e., the distance from the light source 10to the beam shaper 113. If the fiber 114 is placed substantially furtherthan a depth of focus away from the image plane, then the beam will bebigger than the core 116 of the fiber 114, resulting in less light beingcoupled to the fiber 114.

The optical element 112 may direct light away from a center of the core116 of the fiber 114 by, e.g., increasing the angle of light in thecenter of the beam so that light in the center will be incident on theouter edges of the core 116 of the fiber 114 or by delivering no lightto the center of the core. Thus, either no light is delivered to acenter 116 of the fiber 114 or any light which is incident on the center116 of the fiber 114 will be incident at a high enough angle to becoupled into the desired higher order modes.

In addition to the optical element 112, a beam shaper 113 may beprovided in either embodiment. The beam shaper 113 may be integratedwith the optical element 112 on a same surface or on an opposite surfaceof the same structure. The beam shaper 113 may also be closely spaced tothe optical element 112. As shown in FIG. 5B, the beam shaper 113 may beplaced at a specific distance from the light source 110, with theoptical element 112 being very close to or even flush with the fiber114. Each embodiment has attendant advantages and disadvantages asdiscussed below.

The beam shaper 113 performs a one-to-one mapping from the input planeto the output plane thereof. The performance of the beam shaper may beevaluated using ray tracing. Typically, the beam shaper 113 is used forfocusing the beam output by the light source 110, which will usually beon the order of a several hundred microns in the plane of the beamshaper, to a diameter which is smaller than the diameter of the core,which is usually on the order of 50 microns. If the beam shaper is alens, theoretically, light is focused to a point. But in reality, if thelight incident on the lens has a Gaussian profile, the light output fromthe lens will still have a Gaussian profile. Another useful beam shaperfor the coupling of the present invention is a super-Gaussian element. Asuper-Gaussian element converts an input beam of a particular intensitydistribution into a beam with a super-Gaussian distribution, therebyproviding a focused output beam having a flatter peak and a much fasterfall off to zero than a normal Gaussian beam. Thus, such a beam has afairly uniform power distribution across the peak, pushing more power tothe edges and leaving less in the center as compared to a normalGaussian. When the optical element serves as an optical profile alteringelement, the beam shaping and the optical element may be formed on asingle surface.

While the ratio between the distance from the light source 110 to thebeam shaper and the distance from the beam shaper to the fiber 114 shownin FIGS. 5A and 5B, in which an edge emitting laser is used as the lightsource, is typically 2:1, when using a VCSEL as the light source 110,this ratio is typically closer to 1:1. The actual ratio will depend onthe numerical aperture of the source and the numerical aperture of thefiber. Further, depending upon the desired coupling, the beam incidenton the fiber may be smaller than the core or larger than the core. Formost applications, source and fibers, the ratio will be between 1:4 and4:1.

There are three primary design approaches for achieving the desiredshaping by the optical element 112. The radiation profile of lighthaving traversed a first embodiment of the optical element 112 is shownin FIG. 6. As can be seen in FIG. 6, the radiation profile has beenaltered by the optical element 112 to be bimodal. This bimodaldistribution is Gaussian shaped for each peak, each peak being centeredon an absolute angular value between zero and θmax, where θmax is thecritical angle for the multi-mode fiber 114.

In the first design, the optical element 112 is a diffractive diffuserwhich diffuses, i.e., substantially each point of light incident on thediffuser substantially contributes to substantially every point of lightin the output plane, the light into the desired angular distribution.The angles will all be less than the critical angle for the fiber 114.Thus, if there is change in the output profile of the light from thelight source, which is of particular concern when using a VCSEL as thelight source 110, the coupling to the multi-mode fiber will not beaffected. Additionally, if the diffractive diffuser does not alsoprovide collimation or focusing to the light, precise alignment of thediffractive diffuser is not needed.

A diffractive diffuser may be formed by setting the fast Fouriertransform (FFT) to be a ring, i.e., set the fringe period of thediffractive between the two values bounding the ring. In order for thediffractive diffuser serving as the optical element 112 to functionproperly, it must be positioned at least more than a width of the core,preferably at least three to five times the width of the core, away fromthe fiber. This placement ensures that substantially every point oflight incident on the diffuser substantially contributes tosubstantially all of the pattern incident on the fiber. Such a Fouriertransform diffractive diffuser may be realized in accordance with U.S.Pat. No. 5,850,300, which is hereby incorporated by reference in itsentirety.

The diffractive diffuser preferably alters the angular distribution ofthe light into any desired angular distribution which will efficientlycoupler the light into the higher order modes of the multi-mode fiber.This desired angular distribution will typically be a ring, an annulusor a grid of N spots, but may be any other desired angular distributionfor a particular multi-mode fiber. For example, a radial grating may beprovided which sends a significant portion of the light, e.g., 80%, intothe ±1 order and randomly varies the period to provide the range ofdesired angles radially away from the center. Further, a ring or amultipole of N spots, e.g. a quadropole of 4 spots, where N is aninteger greater than or equal to one, may be realized by providing agrating to create spots located at r_(N), where r_(N) is a distance fromthe center to the spot. Additionally, the diffractive diffuser may be abinary element which splits the light into two beams directed to theperiphery of the fiber core.

While a Fourier transform diffractive diffuser as described above isuseful when employing a light source having an unstable output beamprofile, this diffractive diffuser is difficult to use in theconfiguration of FIG. 5B, since the optical element 112 is too close tothe fiber 114 for a Fourier transform diffractive diffuser to create,for example, a ring on the end face of the fiber. If the optical element112 is more than a few wavelengths away from the end of the fiber, adiffuser serving as the optical element 112 will function properly.Additionally, diffusers often have lower efficiencies than other opticalelements. The following two designs may be used with eitherconfiguration of FIGS. 5A and 5B.

A generic embodiment of the second design is illustrated in FIG. 7. Ascan be seen therein, the optical element 112 is composed of a centralregion 118 and a peripheral region 120. The central region 118 and theperipheral region 120 affect the beam incident thereon differently.These different regions may be discretely different, include subregionsof different functioning, and/or may continuously vary the treatment ofthe light from the center to the periphery. For example, the centralregion 118 deflects the light incident thereon away from the center. Theperipheral region 120 may not affect the light incident thereon at all,or it may be designed to, for example, collimate the light incidentthereon. Using such an element allows the light in the center of thebeam which would have been incident on the center of the fiber to bedeflected away to edges of the fiber, while not imposing an increase inthe angle on the light near the edge of the beam which would already beincident upon the desired portion of the fiber. Alternatively, althoughnot as efficiently, the central region may simply block the lightincident thereon to form the desired ring shape.

A specific embodiment of the second design is illustrated in FIG. 8A.The optical element 112 provides a one to one mapping of each point tothe fiber, while continuously varying the element encountered by thelight from the center to the periphery. A converging portion 150 of thecoupler 112 converges, i.e., reduces the incident angle, of light at theouter edge of the beam. A diverging prism 152 of the optical element 112diverges, i.e., increases the angle of the light, of light in the centerof the beam to prevent light from hitting the center of the fiber.Another specific embodiment of one to one mapping is shown in FIG. 8B inwhich a diverging portion 154 is located in the center of the opticalelement again to diverge light in the center of the beam. FIGS. 8A and8B are radially symmetric.

Another specific embodiment of the second design is shown in FIG. 9.FIG. 9 is a cross-section of a prism. If this cross-section is used toform a linear prism, such that there is a variation in thickness alongthe axis coming out of the plane of the page, two spots will begenerated in the image plane of the system. When a linear prism iscombined with a lens function, the cross-section will look like FIG. 8B,but will not be radially symmetric, since the linear prism is notradially symmetric. If the cross-section in FIG. 9 is rotated radiallyto form a radial prism, a ring will be generated in the image plane ofthe system. If the radial prism is combined with a lens function, thecross-section will look like FIG. 8B and will be radially symmetric.

In the embodiments of the second design, light near the edge of the beamcan be mapped to the edge of the fiber with little or no increase in theangle. Light from the center of the beam can be mapped to the edges ofthe fiber. Where the optical element 112 is illustrated as a refractiveelement in the embodiments of the second design, the optical element 112may be designed as a diffractive element using the known diffractiveapproximation of the refractive element, either as a continuousdiffractive or as a discrete diffractive. Preferably, the diffractiveelements are computer generated holograms.

The same effect as provided by configurations of the second design maybe realized by providing an optical element having diffuser patcheshaving finer features and/or smaller periods closer to the center andlarger features and/or larger periods towards the periphery or nothingat the periphery. At the edge of the element the light is not affected,or has a small increase in angle, and the light at the center isdiffused to increase the angle of light towards the center. As long asthe diffuser patches are distributed on the optical element so that itdoes not treat the center and the periphery in the same manner, e.g., adiffuser only at the center or a gradient diffuser, the diffuser patchesmay be used next to the end face of the fiber, such as shown in FIG. 5B.Such diffusing patches may also be multiplexed with any desired lensfunction.

Further, while the embodiments of the second design have been discussedwith reference to the optical element 112, the second design may also beused as the beam shaper with the optical element of the first design orthe optical element of the third design, discussed above. Further, whenusing a diffractive diffuser which splits the light into two beamsdirected to the periphery of the fiber core, this element does not haveto be unitary, but may be split into half. In such a configuration, thetwo elements serve as a beam shaper, with one half mapping the lightincident thereon to one point and the other half mapping the lightincident thereon to another point.

The above discussion has assumed that the ideal radiation pattern forcoupling light into the fiber is a ring. Generally, the ideal radiationpattern, and hence the desired angular distribution, will be a functionof the properties of the fiber, i.e., where propagation is mostefficient. The design of the coupler for achieving the desired angulardistribution will also depend on the radiation profile output by thelight source used to illuminate the fiber.

Any of the above designs may be integrated with other optical functions,such as collimation, in a single element, as shown in FIGS. 8A, 8B and9. For the integration of the coupler with additional opticalfunctioning, the additional functioning may be multiplexed with theshaping function, as disclosed in commonly assigned, co-pendingapplication U.S. patent application Ser. No. 09/296,397 filed Apr. 23,1999, entitled “Diffusing Imager and Associated Methods” which is herebyincorporated by reference in its entirety. Further, any of the abovedesigns may be integrated with the other elements of the lightsource/fiber system, including further optical elements as discussedabove.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the present invention is not limited thereto. Thosehaving ordinary skill in the art and access to the teachings providedherein will recognize additional modifications, applications, andembodiments within the scope thereof and additional fields in which theinvention would be of significant utility without undue experimentation.

1.-5. (canceled)
 6. An optical system comprising: an optical elementhaving an input surface, an output surface, and an optical axis; and anoptical medium having an end proximate to the output surface of theoptical element and an end distal to the output surface of the opticalelement, the optical medium configured to receive emitted light having areceived annular intensity profile and transfer the light to the distalend such that the transferred light substantially retains the receivedannular intensity profile at the distal end; and wherein: the outputsurface has a continuous slope/curvature in an area approximatelybetween a periphery of the output surface and a first distance from theoptical axis; and the output surface has a discontinuity in an areawithin a perimeter of the first distance for emitting light having anoutput annular intensity profile receivable at the proximate end of theoptical medium.
 7. The system of claim 6, wherein the optical element isa mode matching element, wherein light output from the optical elementis distributed in a desired angular distribution which is substantiallymaintained along the fiber for more than a depth of focus of the opticalelement.
 8. The system of claim 7, wherein said mode matching element isa diffractive element.
 9. The system of claim 7, wherein said modematching element is a refractive element.
 10. The system of claim 7,wherein said optical element comprises first and second surfaces, saidmode matching element being provided on said second surface, furtherfrom the light source.
 11. The system of claim 10, further comprising anangular distribution altering element on said first surface.
 12. Thesystem of claim 11, wherein said angular distribution altering elementprovides a ring pattern on said second surface.
 13. The system of claim11, wherein the angular distribution altering element is a diffractivesurface having a radially symmetric lens function and a negative axiconfunction.